(1) Field of the Invention
The present invention relates to an improved acoustic damper used in aircraft engine applications.
(2) Prior Art
A Hemholtz resonator 10, such as shown in FIG. 1, is a device commonly used to dissipate acoustic energy. The resonator 10 typically consists of an aperture 12, an enclosed volume or cavity 14, and a passage or tube 16 connecting the aperture 12 to the volume 14.
Resonators are used in the augmentor section of a gas turbine engine to suppress a thermo-acoustic instability known as screech. If not suppressed, screech has the potential of causing structural damage in the augmentor section. Hemholtz resonators are also used in rocket engines for thermo-acoustic instability suppression and in aircraft nacelle inlets and fan ducts for noise suppression. Maximum acoustic dissipation occurs at the resonant frequency of the Hemholtz resonator, which is determined by the geometric parameters of the resonator, i.e. aperture area, passage length, and cavity volume. The resonant frequency of a resonator can be adjusted by varying its geometric parameters. The performance of the resonator, i.e. the degree of acoustic suppression it provides, is determined by both the geometric parameters of the resonator and the flow resistance (or total pressure loss) that the working fluid encounters when it enters the volume through the aperture.
In an application where the fluid external to the resonator has no mean flow grazing the aperture, the major loss components are the frictional loss in the aperture, and the dump loss encountered by the fluid as it enters the volume. In an application where the external fluid has a mean flow grazing the aperture 12, as shown in FIG. 2, the flow resistance through the aperture 12 increases significantly, compromising the performance of the resonator. The performance of the resonator degrades as the grazing flow Mach number increases. The reason for degradation in resonance performance with grazing flow Mach number is that the flow 18 separates from the leading edge 19 of the aperture, as illustrated in FIG. 2, increasing the flow resistance. The amount of air entering the resonator 10 is limited by the size of the recirculation zone 17. The recirculation zone 17 reduces the size of the available aperture cross section through which fluid can flow in to the resonator 10.